1. Field of the Invention
The present invention relates to a diaphragm for the molten salt bath electrolysis of halides of metals. It relates to all metals which have a plurality of valency states, that is to say the polyvalent metals such as, in particular, titanium, zirconium, hafnium, thorium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, uranium, plutonium and also the rare earth metals.
2. Description of Related Art
A man skilled in the art knows that it is possible to obtain a metal by introducing one of its derivatives such as a halide, for example, into a molten salt bath and subjecting it, in its simplest principle, to the action of two electrodes connected to the poles of a direct current source. Halogen is given off at the anode and the metal is deposited on the cathode. This technique, which is referred to as dry electrolysis, has been the subject of many studies which have resulted in the conception of various processes which are distinguished from one another by the composition of the bath, the physical and chemical state of the halide, the modulation of the current system applied and to the production of multiple apparatuses which differ in terms of structure and shape, particularly where the electrodes, the systems of halide injection and recovery of the deposited metal are concerned.
However, all these cells have one point in common, which is the presence of a porous diaphragm which separates the anode from the cathode in such a way as to divide the bath into two distinct spaces: the anolyte and the catholyte. This diaphragm, which can be electrically polarized, has the effect of avoiding the halogen given off at the anode reoxidizing the reduced halides dissolved in the electrolyte when the metal has several valences.
This diaphragm generally consists either of a metal grid (see, for example, U.S. Pat. No. 2,789,983) or a porous graphite or ceramic member. However, these materials have their drawbacks. For example, the use of metal diaphragms results:
on the one hand, in chemical instability PA1 instability vis-a-vis the bath because the metals are capable of at least partially dissolving in it so that they pollute the metal which is to be deposited; PA1 instability vis-a-vis the halogen released which can corrode it to the point of destroying it locally and eliminating the separating between the anolyte and the catholyte; PA1 instability to the bath-atmosphere interface by electrochemical corrosion, and PA1 instability vis-a-vis the metal deposited by the formation of intermetallic compounds such as, for example, Ti-Ni or TiFe alloys which will render the diaphragm fragile. PA1 on the other, an electrical instability due to the fact that the diaphragm is the focal point of successive depositions and redissolutions of the metal to be deposited which change its porosity and affect maintenance of the optimum electrical deposition conditions; indeed, it is possible to follow the evolution of this porosity by measuring the potential and restoring it to suitable limits by polarization, as described in U.S. Pat. No. 4,392,924. However, the range of potential corresponding to normal running of the cell may be relatively narrow and of the order of 10 mV so that monitoring the porosity is no easy matter and it is easily possible to finish up with either a complete blocking of the diaphragm or an electrochemical attack on the diaphragm which more often than not results in the cell shutting down and the faulty diaphragm having to be replaced. PA1 Relatively high fragility which makes it sensitive to shocks and not suitable for machining operations such as screwthreading, for example, which would be necessary for connecting it to the dome or for the cutting of apertures to ensure the desired porosity; PA1 a troublesome tendency to absorb alkaline compounds from the bath which are inserted into the pores and cause it to burst; PA1 an aptitude to combine with certain metals which have to be deposited to form carbides which, in addition to enhanced fragility, alters its porosity and has a harmful effect on maintenance of optimum electrical conditions for deposition. With regard to ceramic diaphragms, apart from their fragility and their sensitivity to thermal shocks, they have the disadvantage of having very low electrical conductance, which means that they cannot be electrically polarized.
There are just as many factors which help to limit the life of the diaphragm.
Furthermore, the diaphragm is generally extended upwardly and around the anode by a kind of bell or dome which is intended to channel the halogen released. Then, there are problems connected with the linking together of these two members which may give rise to mechanical and electrical difficulties, particularly in the case of polarized diaphragms.
With regard to graphite, it has the advantage over metals of being relatively insensitive to corrosion, but it does, however, have its drawbacks, viz.:
Consequently, they cannot lend themselves to the electrolytic redissolution of deposits which form on their surface so that it is impossible to monitor their porosity which renders them useless, particularly in the case of electrolysis of polyvalent metals.